1. Field of the Invention
The present invention relates to the photometric assay of sample material and, more particularly, to the optical assay of sample in containers subject to optical variations or imperfections.
2. Description of the Prior Art
U.S. Pat. No. 4,136,953 (Klein et al.) and U.S. Pat. No. 4,157,871 (Anderson et al.) disclose nephelometric systems for the assay of antigens and antibodies supported within cylindrical glass containers. In such analysis, an antigen and an antibody are combined in the container and the resulting reaction between these sample components produces a precipitate which increases in quantity and turbidity as the reaction progresses. An excitation system directs a beam of light into the container and a detection system measures light scattered at a designated angle by the precipitate. The light scattered by the precipitate is a function of its turbidity and is employed to derive quantitative or qualitative information about the antigen or antibody sample components.
The above-mentioned Anderson et al. patent discloses a system employing kinetic methods for measuring the change in scatter during an early part of the antigen antibody reaction and for deriving the maximum rate of change of the scattered light to assay the sample components. Using this approach the measurement can be completed rapidly in less than one minute without waiting for the reaction to proceed to completion. In a second known approach, termed endpoint nephelometry, the scattered light intensity is measured twice--first at the beginning of the reaction and again at the end of the reaction. Unfortunately, for some materials, the reaction time from beginning to end is unacceptably long--i.e. from several minutes to several hours or more. Consequently, after the initial scatter measurement is made, the sample container is removed from the nephelometer and placed in an incubation area where the reaction progresses toward completion. The nephelometer is thus freed in the interim for measuring other samples. When the reaction is complete, the container is then repositioned in the nephelometer for the final measurement.
It has been found that imperfections and variations in the sample container wall or window areas, such as scratches, bubbles, deformations, and the like, can introduce significant errors in the measurement of light scattered by the sample--typically by intercepting and attenuating or otherwise distorting light passing therethrough. In addition the nature and extent of these errors will vary from one container to the next. Moreover, the number and severity of imperfections will vary around the circumference of a single container, which causes the measured error for the single container to depend on the relative rotational orientation of the container within the optical system. This compounds the problem for the aforedescribed endpoint measurements, since the container could be and almost invariably is reinserted into the nephelometer for the second measurement at a different rotational orientation than it had been in for the first measurement. Consequently, the error in the measured scattered light signal will be different for the first measurement than for the second measurement on the same sample. Of course, this error difference could be eliminated by configuring the container or by employing a mechanical key integral with the container enabling container placement in the nephelometer in only one rotational orientation. However, this expedient would not reduce the measured error differences between different containers. Moreover, a primary reason for using the disposable, cylindrical glass containers described above is their economy, simplicity and convenience, and any increase in structural or mechanical complexity would reduce their attractiveness from this standpoint.